Ok, it sounds like you're doing everything the way I would have suggested. I heat prey in water too. I find the extra hydration can't hurt.
Some are just smaller than others, some grow slower than others. Without knowing more about her background (parents sizes, hatch weight, how soon they started eating as a hatchling, etc...). As long as she's maintaining a good body structure I wouldn't sweat it.
Perfect example of how different things work for different people and different animals. There really are no "one size fits all" answers.
I stopped feeding my BPs on a schedule because I was getting refusals. Since I started basically letting them "tell" me when they're hungry (by observing behavior at night) I haven't had any refusals and they both eat year round.
Silly little BPs.